Process for producing aliphatic carboxylic acids and aromatic carboxylic acids

ABSTRACT

A PROCESS FOR THE SIMULTANEOUS PRODUCTION OF AN ALIPHATIC CARBOXYLIC ACID AND AN AROMATIC CARBOXYLIC ACID IN A HIGH YEILD BY OXIDIZING AN ALIPHATIC SECONDARY ALCOHOL AND AN ALKYL- SUBSTITUTED AROMATIC HYDROCARBON IN THE LIQUID PHASE IN AN AMOUNT OIF FROM 0.05 TO 1.0 TIMES THE WEIGHT OF SAID ALIPHATIC SECONDARY ALCOHOL AT A TEMPERATURE OF FROM 100 TO 160*C. AND AT A PARTIAL PRESSURE OF OXYGEN OF LOWER THAN 3 KG./SQ. CM. IN THE PRESENCE OF A COBALT COMPOUND.

Dec. 11, 1973 TAKESHI YAMAHARA ETAL 3,778,471

PROCESS FOR PRODUCING ALIPHATIC CARBOXYLIC ACIDS AND AROMATIC CARBOXYLICACIDS Filed Dec. 12, 1969 APPARENT VOLUME OFOXYGEN ABSORBED TIME (HRS)INVENTORS TAKESHI YAMAHARA TAKASHI DEGUCHI g R w, M I

ATTORNEYS United States 3 Patent US. Cl. 260-524 R Claims ABSTRACT OFTHE DISCLOSURE A process for the simultaneous production of an aliphaticcarboxylic acid and an aromatic carboxylic acid in a high yield byoxidizing an aliphatic secondar alcohol and an alkyl-substitutedaromatic hydrocarbon in the liquid phase in an amount of from 0.05 to1.0 times the weight of said aliphatic secondary alcohol at atemperature of from 100 to 160 C. and at a partial pressure of oxygen oflower than 3 kg./sq. cm. in the presence of a cobalt compound.

BACKGROUND OF THE INVENTION (1) Field of the invention The presentinvention relates to a process for producing simultaneously an aliphaticcarboxylic acid and an aromatic carboxylic acid by oxidizingsimultaneously an aliphatic secondary alcohol and an alkyl-substitutedaromatic hydrocarbon in the liquid phase with an oxygencontaining gas.

(2) Description of the prior art It is generally diflicult to prepare acarboxylic acid by the oxidative decomposition of an aliphatic secondaryalcohol and. thus the so-called nitric acid oxidation method has beenemployed for the purpose. However, in modifying the method usingmolecular oxygen, quite severe conditions are required and just as theconventional method results in a low yield.

Also, a method for preparing acetic acid involving the liquid-phaseoxidation of the acetic acid ester of the secbutanol with oxygen insteadof the direct oxidation of the alcohol is disclosed in Japanese patentpublication No. 9,173/66. This method also requires severe conditionssuch as a temperature of 190 C. and a pressure of 80 atms.

On the other hand, when an aromatic hydrocarbon contains two or moreoxidizable alkyl groups, one of the alkyl groups can be oxidizedcomparatively readily to a carboxyl group, while the monocarboxylic acidthus formed is essentially resistant to further oxidation into the diorpolycarboxylic acid.

Accordingly, a method wherein a monocarboxylic acid is converted intothe methyl ester and then the methyl ester is oxidized, a method whereinan aromatic hydrocarbon is oxidized under severe conditions in thepresence of a catalyst containing bromine, and a method wherein anaromatic hydrocarbon is oxidized in the presence of a small proportionof an oxidation accelerator have been proposed.

For example, Japanese patent publication No. 9,654/ 66 (US. patentapplication Ser. No. 314,498, filed Oct. 7, 1963), a method employing anoxidation accelerator, teaches a method of adding sec-butanol to thereaction system in a small amount in comparison with the amount of lowerfatty acid employed as solvent. However, when this method is applied tothe oxidation of p-xylene, the yield of terephthalic acid thus obtainedis low as only 77.6% and yet necessitates a partial pressure of oxygenas high as from 7.03 to 70.3 kg./ sq. cm., making the methodeconomically unprofitable and unsafe. It has been found byexperimentation that, in the reaction according to the above patent, thesec-butanol added to the reaction system is consumed during the processthereof.

Therefore, it is an object of the present invention to provide anindustrially profitable process for producing simultaneously anindustrial and useful aliphatic carboxylic acid and an aromaticcarboxylic acid by oxidizing a secondary alcohol and analkyl-substituted aromatic hydrocarbon with an oxygen-containing gas inthe presence of a cobalt compound.

SUMMARY OF THE INVENTION Accordingly, the process of this invention forproducing an aliphatic carboxylic acid comprises oxidizing an aliphaticsecondary alcohol with an oxygen-containing gas and procedures analiphatic carboxylic acid with a high yield under very mild conditions.More precisely, the process of this invention comprises contacting analiphatic secondary alcohol and an alkyl-substituted aromatichydrocarbon, in an amount of from 0.5 to 1.0 times by weight based onthe weight of the aliphatic secondary alcohol, with an oxygen-containinggas in the presence of a cobalt-compound. Further, the alkyl-substitutedaromatic hydrocarbon is converted into an aromatic carboxylic acidsimultaneously in high yield.

The merits of the process of the present invention are as follows:

(1) By oxidizing an aliphatic secondary alcohol and an alkyl-substitutedaromatic hydrocarbon simultaneously, the oxidation reaction can beconducted under very mild conditions, conditions so mild that oxidationof each component separately is difficnlt to accomplish, andindustrially useful aliphatic carboxylic acids such as acetic acid andpropionic acid and aromatic carboxylic acids such as benzoic acid,terephthalic acid, isophthalic acid, and phthalic acid can be obtainedwith high yields.

(2) The carboxylic acids produced are obtained in such purity thatfurther purification procedures are almost unnecessary.

(3) Production of an aliphatic carboxylic acid and an aromaticcarboxylic acid simultaneously is economically profitable.

(4) Because the oxidation reaction of the present invention is conductedunder mild conditions, the reaction conditions can be controlled readilyand the reaction can be carried out quite safely.

DETAILED DESCRIPTION OF THE INVENTION Examples of suitable aliphaticsecondary alcohols employed in the process of this invention areisopropanol, sec-butanol, sec-pentanol, and the like. Examples ofsuitable alkyl-substituted aromatic hydrocarbons used in the presentinvention are toluene, ethylbenzene, cumene, o-xylene, m-xylene,p-xylene, o-ethyltoluene, m-ethyltoluene, p-ethyltoluene, o-cymene,m-cymene, p-cymene, o-diisopropylbenzene, m-diisopropylbenzene,p-diisopropylbenzene, mesitylene, and other polyalkylbenzenes.Furthermore, as a matter of course, such alkyl-substituted aromaticcarboxylic acid, as o-, m-, and p-toluic ac ds and their esterderivatives can be used in the present invention.

In the process of the present invention, by varying the feed ratios ofthe alkyl-substituted aromatic hydrocarbons to the aliphatic secondaryalcohol, the ratio of the aromatic carboxylic acid to the aliphaticcarboxylic acid produced can be varied as desired. A preferred feedratio of the alkyl-substituted aromatic hydrocarbon to the aliphaticsecondary alcohol is from 0.05 to 1 by weight. If the feed ratio ishigher than 1, the yield of the aromatic carboxylic acid is reduced andalso the yield of the aliphatic carboxylic acid is extremely reduced.Conversely, if the feed ratio is lower than 0.05, the reaction proceedswith difiiculty.

In the process of the present invention, the presence of thealkyl-substituted aromatic hydrocarbon has an important role in theoxidation of the aliphatic secondary alcohol because when thealkyl-substituted aromatic hydrocarbon is absent in the reaction system,the oxidation of the secondary alcohol scarcely proceeds as shown inComparison Example 1, given hereinafter. However, the material to beprofitably oxidized in the presence of the alkyl-substituted aromatichydrocarbon is the hereinbefore described aliphatic secondary alcohol.If the secondary alcohol is supplied to the reaction system as analiphatic acid ester thereof, an aliphatic carboxylic acid is notobtained or is obtained in an extremely low yield and the yield of thearomatic carboxylic acid is low correspondingly. This is illustrated inComparison Example 2 given hereinafter.

The process of the present invention can be conducted elfectivelywithout using a solvent but it is preferably conducted using thealiphatic carboxylic acid, which is one of the products of the processof this invention, as the solvent.

In the process of this invention, a cobalt compound is employed ascatalyst. The cobalt compounds which are preferably used are thosecobalt compounds which are soluble in the reaction liquid, such ascobalt acetate, cobalt tolylate, cobalt naphthenate, and the like. Othercobalt compounds can also be employed where desired.

The reaction of the present invention is carried out at normal pressuresor under raised pressure. Also, the process of the present invention canbe practiced at an oxygen partial pressure in a range from 0.01 to 30kg./ sq. cm. The process of the present invention can be carried outwith a sufiiciently high rate of reaction even at low oxygen partialpressures, which facilitates the control of the reaction conditions andprovides a high degree of safety. On considering these points, apreferred oxygen partial pressure in the process of this invention isless than 3 kg./ sq. cm. Although the reaction of the present inventionproceeds with a high yield even at an oxygen partial pressure higherthan 3 kg./sq. cm., when the oxygen partial pressure becomes higher than3 kg./sq. cm., a mild reaction in the beginning is followed by a violentreaction, which makes the removal of reaction heat and the control ofreaction temperature more difficult. Also, in such case, side reactioncan occur. Moreover, with the employment of a high oxygen partialpressure the control of the composition of the vapour phase in thereaction system outside of the explosive range is difiicult, whichincreases the danger of explosion together with the occurrence of theviolent reaction. On the other hand, while the lower limit of the oxygenpartial pressure is not critical, it is preferably higher than 0.01 kg./sq. cm. A most preferred partial pressure is from 0.1 to 3 kg./sq. cm.

The reaction temperature in the process of this invention can range from100 C. to 160 C. If the temperature is lower than 100 C., the reactionproceeds slowly, which makes the process industrially unprofitable.Whereas, if the reaction temperature is higher than 160 C., sidereactions can occur to reduce the yield of carboxylic acids as well asreduce the quality of the products.

Thus, the most preferred combination of reaction temperatures and oxygenpartial pressures is from 100 to 160 C. in reaction temperature andlower than 3 kg./sq. cm. in oxygen partial pressure. Under theseconditions, the absorption rate of oxygen is essentially constant duringthe entire reaction time and also the reaction proceeds with asufl'iciently high rate of reaction.

As a result, in the process of this invention, the removal of the heatof reaction and the control of the reaction temperature can be quiteeasily conducted, the desired carboxylic acids can be obtained in a highyield using a comparatively small reactor, and also the process can beoperated safely.

As an oxygen-containing gas employed in the process of this invention,there can be used a highly enriched oxygen gas or a gas mixture ofoxygen and an inert gas, such as nitrogen or carbon dioxide. In general,air is most suitably use. Air enriched with oxygen can be used wheredesired.

In the process of the present invention, the aliphatic carboxylic acidcan be separated from the aromatic carboxylic acid produced byconventional means, such as filtration, distillation and the like afterthe reaction is completed. The carboxylic acids, thus prepared, havehigh purity.

Furthermore, the residues left after separating the carboxylic acidsfrom the reaction product mixture can be adjusted to the propercomposition and used again in the oxidation reaction.

Thus, by repeatedly using the unreacted materials or partially oxidizedproducts, the yields of the desired products can be improved further.

The process of this invention can be conducted as a batch system orcontinuously.

The present invention is illustrated below by reference to examples andthe attached drawing. The examples are merely illustrative and are notto be interpreted as limiting. Various modifications are possible withinthe scope of the invention. In the examples, all percents are percentsby weight unless otherwise indicated.

Reference is made to the attached drawing. The attached drawing is agraph showing the apparent volume of oxygen absorbed (1) during thecourse of the reaction proceeding according to the process of thisinvention. The curves labeled Example 4, Example 6 and Example 7 showthe apparent volumes of oxygen absorbed at specific time periods duringthe course of the reaction performed in these examples.

EXAMPLE 1 In a flask equipped with a stirrer, a thermometer, an inletfor oxygen gas, and a reflux condenser were charged 42.5 g. of p-xylene,59.3 g. of sec-butanol, 288 g. of acetic acid, and 9.96 g. of cobaltacetate tetrahydrate. The flask was immersed in an oil bath at C., anoxygencontaining gas was passed into the system in a rate of 5liters/hr. while stirring the system. After the passing of the oxygengas was started, absorption of oxygen occurred immediately and theabsorption was finished after 25 hours, when the amount of oxygenabsorbed was 52 liters.

After cooling the reaction mixture, a solid product thus formed wasseparated by filtration, washed with acetic acid and further washed withether to provide 61.0 g. of a pure white powder of terephthalic acidwith a yield of 91.8%. Also, from the filtrate was recovered 2.5 g. ofptoluic acid. By analyzing the reaction product liquid, the entireacetic acid content in the liqued was confirmed to be 328.8 g.Furthermore, the conversion of sec-butanol was 61.3% and the selectivityfor acetic acid was 68%.

COMPARISON EXAMPLE 1 The same procedure as in Example 1 was followedwith the exception that p-xylene was not added. No absorption of oxygenwas observed even after 8 hours.

COMPARISON EXAMPLE 2 The same procedure as in Example 1 was followedwith the exception that 92.9 g. of the sec-butyl acetate was usedinstead of the alcohol and the amount of acetic acid was changed to 240g. After 32 hours, the absorption of oxygen was stopped. Using theprocedures of Example 1, 53 g. of terephthalic acid was obtained. Thesec-butyl acetate yield was 15;% and no acetic acid was obtained.

EXAMPLE 2 In a flask as in Example 1 were charged 42.5 g. of pxylene,118.6 g. of sec-butanol, 240 g. of acetic acid and 7.1 g. of anhydrouscobalt acetate. The flask was immersed in an oil bath at 100 C., andoxygen gas was passed into the system at a rate of 5 liters/hour withstirring. The absorption of oxygen was completed after 51 hours andusing the procedures of Example 1, 60.5 g. of a white powder ofterephthalic acid were obtained in a yield of 91%. Also, the amount ofacetic acid in the reaction product liquid was 335.9 g. The conversionof secbutanol was 67.0% and the selectivity for acetic acid was 70.5%.

EXAMPLE 3 In a 300 ml. stirring-type autoclave were charged 8.6 g. ofp-xylene, 16.0 of sec-butanol, 10.4 g. of acetic acid, 6.0 g. of cobaltnaphthenate, and 50 ml. of benzene and then the system was heated to 130C., with stirring, under an oxygen pressure of 30 kg./sq. cm. Afterhours, the autoclave was cooled and the reaction product was filtered toprovide 9.7 g. of terephthalic acid with a yield of 79%. The conversionof sec-butanol was 80% and the selectivity for acetic acid was 58%. Fromthe filtrate was recovered 1.3 g. of p-touic acid.

EXAMPLE 4' In a stirring-type stainless steel autoclave were charged8.62 g. of p-xylene, 23.98 g. of sec-butanol, 52.23 g. of acetic acid,and 2.0 g. of cobalt acetate tetrahydrate. The autoclave was immersed inan oil bath at 120 C. and an oxygen-containing gas was introduced intothe autoclave, with stirring to a pressure of 30 kg./sq. cm. Thereafter,the oxygen gas was supplied to the system so that the partial pressureof oxygen was maintained at 30 kg./ sq. cm. After 3 hours, theabsorption of oxygen was stopped. The absorption characteristics ofoxygen of the example are shown by the curve labeled Example 4 in theaccompanying drawing.

After cooling, the reaction product mixture, the solid product, thusformed, was separated by filtration, washed with acetic acid and driedto provide 12.68 g. of pure white powder of terephthalic acid. Theamount of acetic acid in the-reaction product liquid was 80.1 g. and theselectivity of acetic acid per the sec-butanol reacted was 82.1%.

EXAMPLE 5 In the reaction vessel as used in Example 4 were charged 17.24g. of p-xylene, 31.97 g. of sec-butanol, 52.23 g. of acetic acid, and2.00 g. of cobalt acetate and then the same procedure of Example 4 wasconducted for 190 minutes. Terephthalic acid was obtained with a yieldof 94.5%. The conversion of sec-butanol was 88.9% and the selectivityfor acetic acid was 86.0%.

COMPARISON EXAMPLES 3 and 4 The same procedure as used in Example 4 wasrepeated using 17.24 g. of p-xylene, 52.23 g. of acetic acid, and 2.00g. of cobalt acetate for Comparison Example 3 and using 17.24 g. ofp-xylene, 7.99 g. of sec-butanol, 52.23 g. of acetic acid, and 2.00 g.of cobalt acetate for Comparison Example 4. The oxidation reaction wasconducted for 180 minutes for Comparison Example 3 and for 210 minutesfor Comparison Example 4. The yield for terephthalic acid was 30.2% inComparison Example 3 and 66.6% in Comparison Example 4. Also inComparison Example 4, the conversion of sec-butanol was 88.9% and theselectivity for acetic acid was 5.0%.

EXAMPLE 6 In a 200 ml. stirring-type autoclave equipped with an inletfor oxygen gas and a reflux condenser were charged 8.60 g. of p-xylene,24.0 g. of sec-butanol, 52.2 g. of acetic acid, and 2.00 g. of cobaltacetate tetrahydrate and the autoclave was immersed in an oil bathmaintained at temperature of C. While stirring the system vigorously anoxygen gas was introduced into the autoclave so that the total pressureof the system was maintained at 2 kg./sq. cm. The gas in the autoclavewas discharged through the reflux condenser at a rate of 3.5 liters/min.(at 25 C. and 1 atm.). After the oxygen gas introduction was begun, theabsorption of oxygen occurred immediately. The apparent absorption rateof oxygen was 4.6 liters/ hr. (at 25 C. and 1 atm.) almost throughoutthe process. This is shown by the curve labeled Example 6 in theaccompanying drawing. After 5 hours, the product was cooled, recoveredby filtration, washed thoroughly with acetic acid, and dried to provide12.54 g. of pure white powder of terephthalic acid in a yield of 93.2%.The analytical results of the reaction product liquid showed that theconversion of sec-butanol was 85.4% and the selectivity for acetic acidwas 81.8%.

EXAMPLE 7 The same procedure as Example 6 was repeated while the totalpressure of the system was changed to 5 kg./ sq. cm., the initial oxygenabsorption rate was as low as 2.6 liters/hour and the rate was increasedgradually up to 17.6 liters/hour. This is shownby the curve labeledExample 7 in the accompanying drawing. Using the procedures of Example6, terephthalic acid was obtained in a yield of 92.5%. The conversion ofsec-butanol was 88.3% and the selectivity for acetic acid was 75.8%.

EXAMPLE 8 The same procedure of Example 6 was followed with theexception that air was employed instead of oxygen gas, the totalpressure in the system was maintained at 10 kg./sq. cm., and the gas inthe autoclave was discharged at a rate of 30.2 liters/hour. The apparentabsorption rate of oxygen was 4.3 liters/hour throughout the wholereaction period. Analysis of the discharged gas showed that the contentof oxygen was 89% by volume.

After conducting the reaction for 5 hours, the reaction product wasworked up as in Example 6. The yield of terephthalic acid thus obtainedwas 93.1%, the conversion of sec-butanol was 84.8%, and the selectivityfor acetic acid was 80.7%.

One g. of the terephthalic acid thus prepared was dissolved in 25 ml. of14% aqueous ammonia. The optical density of the solution was measured ata wave length of 380 m and a cell length of 5 cm. The optical densitythus measured was 0.143.

EXAMPLE 9 The same procedure of Example 8 was followed with theexception that 31.1 g. of acetic acid was employed instead of 52.2 g.The apparent absorption rate of oxygen was 5.9 liters/hour throughoutthe entire reaction period. The oxygen content in the discharged gas was58% by volume. After conducting the reaction for 5 hours, the reactionproduct was worked up as in Example 6, and the terephthalic acid wasobtained in a yield of 94.3%. The conversion of sec-butanol was 84.5%and the selectivity for acetic acid was 85.8%.

EXAMPLE 10 The same procedure of Example 8 was followed with theexception that the amount of sec-butanol and acetic acid were changed to16.0 g. and 31.1 g. respectively. The apparent absorption rate of oxygenwas 6.2 liters/ hour and the oxygen content in the discharged gas was47% by volume. After conducting the reaction for 5 hour and the oxygencontent in the discharged gas was terephthalic acid was obtained in ayield of 95.2%. The conversion of sec-butanol was 88.9% and theselectivity for acetic acid was 82.2%.

What is claimed is:

1. A process for the preparation of an aliphatic carboxylic acid and anaromatic carboxylic acid which comprises contacting a reaction mixtureof an aliphatic secondary alcohol and an alkyl-substituted aromatichydrocarbon with a gas selected from the group consisting of oxygen andoxygen-containing gases in the presence of a cobalt compound as acatalyst, said alkyl-substituted aromatic hydrocarbon being present inthe reaction mixture in a ratio by weight of from 0.05:1 to 1:1 to thealiphatic secondary alcohol.

2. The process of claim 1 wherein said aliphatic secondary alcohol isselected from the group consisting of iso-propanol, sec-butanol, andsec-pentanol.

3. The process of claim 1 wherein said alkyl-substituted aromatichydrocarbon is p-xylene, and said aliphatic secondary alcohol issec-butanol.

4. The process of claim 1 wherein said cobalt compound is selected fromthe group consisting of cobalt acetate, cobalt naphthenate or cobalttolylate.

5. The process of claim 1 wherein the gas is selected from the groupconsisting of mixtures of nitrogen and oxygen, mixtures of carbondioxide and oxygen, air, and air enriched with oxygen.

6. The process of claim 1 wherein the process is conducted at atemperature of from 100 C. to 160 C. and wherein the gas is used in anamount such that the oxygen partial pressure ranges from 0.01 to 30kg./sq. cm.

7. The process of claim 1, wherein the process is conducted at atemperature of from 100 C. to 160 C. and wherein the gas is usedin anamount such that the oxygen partial .pressure is lower than 3 kg./ sq.cm.

8. The process of claim 1 wherein the process is carried out in thepresence of an aliphatic carboxylic acid as a solvent.

9. The process of claim 8 wherein the solvent is the same as thealiphatic carboxylic acid produced in the process.

10. A process for the production of acetic acid and terephthalic acidwhich comprises contacting a reaction mixture of sec-butanol andp-xylene with a gas selected from the group consisting of oxygen andoxygen-containing gases in the presence of a cobalt compound, saidpxylene being present in the reaction mixture in a weight ratio of from0.05:1 to 1:1 to the sec-butanol.

References Cited UNITED STATES PATENTS 3,284,493 11/ 1966 Chibnik 2605243,215,733 11/1965 MacLean et a1. 260524 2,818,428 12/ 1957 Wirth 2605312,883,421 4/1959 Speer 260-524 JAMES A. PATTEN, Primary Examiner

